As a laser device for generating the so-called femtosecond laser which is high-powered (high-energy) laser light having a pulse width on the order of femtoseconds to picoseconds, a laser device using a chirped pulse amplification (CPA) method is commonly known (for example, see Patent Literature 1). In a laser device using the CPA method, the duration (pulse width) of the laser light generated by a laser oscillator is increased by a laser stretcher to reduce its peak intensity, after which its power is amplified by a laser amplifier to such an extent that the laser medium will not be damaged. Subsequently, the laser pulse is sent into a pulse compressor which works opposite to the pulse stretcher and temporally compresses the pulse to produce an output pulse whose peak intensity is increased by an amount corresponding to the amount of compression.
The laser pulse generated by a laser oscillator has an extremely small yet certain amount of wavelength width. The pulse stretching and pulse compression makes use of this wavelength width (margin) to stretch and compress the pulse width (duration) of the laser light. For example, the pulse stretching is achieved by spatially dispersing the light into different wavelength components and making them respectively travel different optical path lengths. The pulse compression is achieved by the opposite principle. In general, a grating pair consisting of a pair of diffraction gratings having the same structure is used in such a pulse stretcher or pulse compressor.
Patent Literature 2 discloses a typical pulse compressor (double-path configuration) employing a grating pair. That is to say, this pulse compressor includes two diffraction gratings arranged parallel to each other with their respective grating surfaces facing each other (i.e. the grating pair) and a mirror for receiving a laser pulse which has passed through the grating pair and for totally reflecting the pulse back to the same grating pair. A laser pulse amplified by a laser amplifier is introduced into this pulse compressor and travels through the grating pair. After being reflected by the mirror, the laser pulse once more travels through the grating pair back to the original point (i.e. the pulse travels a complete cycle). During this process, its wavelength width is reduced, whereby its peak intensity is increased. Such a configuration of the pulse compressor is characterized in that the two gratings can be placed at a comparatively small distance from each other. This is advantageous for reducing the device size.
As just described, in a laser device employing the CPA method, a high-energy laser pulse amplified by a laser amplifier hits the diffraction gratings which constitute the pulse compressor. Therefore, the damage to the grating surfaces (i.e. reflecting surfaces) of the diffraction gratings will be a problem. This means that the damage threshold (i.e. energy tolerance) of the diffraction gratings used in the pulse compressor imposes a considerable restriction in realizing a high-power laser device using the CPA method. To improve the damage threshold of the diffraction gratings used in the pulse compressor, various methods have been proposed, such as increasing the thickness of the reflection coating on the surface of the diffraction grating or using a multilayer dielectric film having a high level of energy tolerance as the reflection coating (see Patent Literature 3 and Non Patent Literature 1). Those methods are certainly effective for improving the damage threshold. However, those techniques increase the production cost of the diffraction gratings and make the laser device more expensive.